Slow SMTP connections, delay after HELO

We just migrated to cPanel from Plesk, and we are having some email related issues. One is the speed in which SMTP transactions are occuring. They are very slow, to the point of timing out for Gmail and a few other things. I have narrowed it down to a delay between the sending of a HELO command and the display of the banner.

So if I do:

telnet myserver.com 25

and then when it connects, I type

helo mydomain.com

It waits 15 seconds before showing the banner. I turned on all logging for Exim, and I get something like this:

You can see the 15 second delay between the first and second lines. I have found other logs files that people have posted for related issue, but the delay they are seeing between the first and second lines is much smaller, like 1 second or a few seconds.